Incident light in an ultraviolet region used in an exposure process would pass through a glass substrate and is undesirably reflected off an upper surface of a vacuum chuck. Since this reflected light causes irregularities in exposure, low reflectance is desired of the vacuum chuck. Within the incident light, a proportion of light that is reflected at the same angle as the incident angle is called regular reflection, or specular reflection. It can be expected that if the surface roughness of the upper surface of the vacuum chuck becomes large, regular reflection will become small. Patent documents 1-3 disclose substrate holding devices that lower regular reflection by roughening a vacuum chuck upper surface. However, roughening sometimes makes diffuse reflection larger. An amount of reflection with diffuse reflection added to regular reflection is called total reflectance or cumulative reflectance. In particular, exposure devices for LCDs of recent years aim for low total reflectance.
In general, a vacuum chuck is formed from black anodized aluminum, or ceramics. Patent document 4 describes that total reflectance of black anodized aluminum is 6-8% in the 200 nm to 650 nm light wavelength range. However, a black and thin-film is subjected to abrasion or peel-off due to repeated chucking of a glass substrate. Also, finish machining process for the vacuum chuck after the black anodizing process has limitations in terms of protecting the black thin-film. Further, uniformly forming a darkened thin-film on a large vacuum chuck is not a simple matter.
Patent document 5 discloses colored alumina ceramics appropriate to a support platform for large meter-size substrates. Regular reflectance of the disclosed appropriate colored ceramics is 0.6-0.9% for light wavelengths of 220-350 nm, while total reflectance is 10.3-22.5% for light wavelengths of 220-350 nm.
In many cases, numerous protruding pins for supporting a glass substrate are formed on the vacuum chuck in order to make the surface area of the vacuum chuck that contacts the glass substrate small. A number of vacuum vents communicating with a vacuum source are formed in a non-contact surface of the vacuum chuck. In order to improve vacuum efficiency a space formed between the non-contact surface and the glass substrate is partitioned by a protruding rim. The rim is also called a bank or dike. The vacuum vents and most of the pins are enclosed by the rim. The rim normally has almost the same height as the pins.
Normally, the pins and the rim are formed using shot blasting following formation of a mask pattern. Hard abrasive grains, such as green carborundum and white alundum, which contain alumina (Al2O3), silicon carbide (SiC), silicone nitride (Si3N4) or Zirconia (ZrO2) as a main component are used in the shot blasting. Depressed area formed in this way is different in total reflectance from the protruding areas. Also, even for the same protruding areas total reflectance of the rim will be slightly higher than that of the pins due to differences in surface area ratios of projections with respect to peripheral depressed area.